The surviving crewman states on Page 20 that he saw the pilots attempting to secure the PCLs. I guess the vibrations prevented them doing so in time.
Hence my comment about doing it really quickly - in ground resonance the amplitude and severity of the oscillations increase very rapidly so if you have dithered whilst trying to diagnose the problem, you are already on your side.
Only good awareness amongst the Wessex fleet about the problem and good training by the RAF prevented me from ending up, like them, as the owner of a big pile of scrap.